1. Technical Field
The present invention relates to a ballast circuit used to ignite a ceramic high intensity discharge (HID) lamp and, more particularly for an improved ignition control of a ceramic high intensity discharge lamp.
2. Description of Related Art
A high-intensity discharge (HID) lamp produces light by means of an electric arc between electrodes housed inside an arc tube of a transparent material such as fused quartz or alumina. The tube is filled with both gas and a dose of metal salts. The gas facilitates an initial strike or ignition of an arc. Once the arc is started, the arc heats and evaporates the metal salts. A plasma is formed which greatly increases the intensity of light produced by the arc and reduces power consumption. In typically 1 to 2 minutes, a low powered 70 W HID lamp warms up to produce its rated light output. When the HID lamp is initially cool, an ignition voltage of 4000 volts for instance is typically required to ignite the HID lamp. After ignition, the HID ballast provides alternating current to the lamp at low voltage, e.g. 20-200 Volts. The physical properties of an HID lamp typically determine the operating voltage across the HID lamp.
Reference is now made to FIG. 2 which shows a plan cross-sectional view of ceramic HID lamp 14, according to conventional art. Ceramic HID lamp 14 includes electrodes 20 extending at the proximal ends of electrodes 20 into an arc chamber 28 which is interior to an arc tube 26. Electrodes 20 connect electrically at the distal ends to the output of the ballast circuit supplying the lamp 14. Electrodes 20 pass through respective bores 24 of capillaries 25 and are sealed inside a portion of bore 24 by seal 22 near the distal ends of capillaries 25.
Lamp 24 of construction as shown in FIG. 2 is known as a “ceramic” HID lamp in distinction with a “quartz” HID lamp with arc tube of material fused silica or polycrystalline quartz. In quartz HID lamps, the seal to the electrode is formed by pinching the tube material while in a viscous semi-liquid state onto the electrode near the entrance to the arc chamber. In ceramic HID lamps of ceramic materials other than fused silica or quartz, the pinch seal is not available and seal 22 is formed by melting glass or ceramic frit inside distal portions of bore 24 within capillaries 25. U.S. Pat. Nos. 7,701,142, 7,728,495 and US patent application publication US20020145388 are representative references describing ceramic arc lamps.
Ceramic HID lamps 14 may provide improvements over the quartz metal halide (MH) lamps, both in the light efficacy, color temperature and color rendering index (CRI). Normally the color temperature of MH quartz lamps is over 4000 kelvin with CRI of 65 to 70. Ceramic lamps 14 typically may provide warmer light typically around 3200K with CRI of 90. Light efficacy may be over 110 lumen per Watt (L/W), while from quartz MH lamps the efficacy is typically around 90 L/W.
Most HID lamps, including ceramic HID lamps 14 are operated at low frequency of less than 400 Hz. However, operating at high frequency range, over 100 kHz may provide advantages such as longer life, lower lumen depreciation, stable color and CRI. For instance, lumen depreciation of a quartz metal halide (MH) lamp at low frequency operation after 8000 hours may go down as low as 50% of the initial value, while operating the same lamp at high frequency may show lumen depreciation of only less than 15% at 8000 hours of operation. Ignition with high frequency may improve even further HID lamp performance over life time comparing to the conventional low frequency ignition methods.
Reference is now made to FIG. 3 which shows two measurement traces 30 for vertical axes voltage (V) and current (I) versus common horizontal time axis for a standard high frequency ignition process normally used for a quartz HID lamp and applied to ceramic HID lamp 14. The initial voltage applied at a time indicated by line 32a is approximately 4000 volts peak-to-peak followed by 2500 volts peak to peak. The time interval between dotted lines 32a and 32b shows a glow-to-arc transition during a time interval of almost 600 milliseconds. In most cases, during the glow-to-arc transition an arc in bore 24 may be observed in bore 24 as bore 24 is being lit up by the glow.
The arc created in bore 24 while lighting up bore 24 was found to be a destructive phenomenon. During the ignition of a cold ceramic HID lamp 14, at high frequency, the metal halides that were condensed in bore 24 provide a low impedance for the arc to build up in the capillaries 25. Only after vaporization of the metal halides and warm up of electrodes 20, the arc move to the proximal ends of electrodes 20 creating stable arc transition from glow. During the glow in the capillaries 25 in bore 24, the energy provided is very high, since the voltage is high (over 300V), although the current is limited to the normal warm up current set-up. This high power can damage or even melt the sealed material in bore 24, and overheat the ceramic tube bore walls of capillaries 25, which may eventually result in catastrophic early failure of ceramic HID lamp 14.
UK patent GB2477463 of the present Applicant discloses application to the electrodes of multiple pre-ignition voltage bursts adapted to avoid arcing in bores 24 of the capillaries surrounding electrodes 20 prior to ignition. The pre-ignition bursts momentarily ignite the ceramic HID lamp and cause significant current to momentarily flow. During the pre-ignition bursts an arc is formed but substantially only between the proximal ends of the electrodes and not in the bore. The ignition circuit is configured by the microprocessor to apply to the electrodes between three and ten pre-ignition bursts, each pre-ignition burst followed by a time delay 0.5-1.5 seconds of substantially zero voltage. The pre-ignition voltage bursts have a previously determined time period set between five to two hundred milliseconds, a peak voltage of 2000-4000 volts and a frequency of 100-500 kilohertz. The pre-ignition voltage bursts heat electrodes 20 prior to normal operation to avoid arcing in the bores of the capillaries surrounding electrodes 20.
Although application of the pre-ignition bursts according to the teachings of GB2477463 were found to successfully ignite ceramic discharge lamps while avoiding arcing in the bores of the capillaries surround the electrodes, the Applicant had limited success in previously determining the duration of the bursts and the time to stop applying the bursts to avoid arcing in bores 24 in a wide range of lamp types and over multiple production runs of the same lamp type.
Thus there is a need for and it would be advantageous to have a control system and method for application of pre-ignition bursts prior to subsequent operation of ceramic HID lamps 14 which determines the burst width and duration of the train of the pre-ignition bursts for different lamps.